Powdered metal technology provides the opportunity to produce articles from mixtures of metals that could not otherwise be prepared by the traditional method of mixing molten metals together. To produce the powders, alloys or pure metals are first melted and the molten stream is atomized by water jets to produce irregularly-shaped metallic particles. Because of their irregular shape, particles of powdered metal have good compactability which facilitates their subsequent formation into finished articles.
Powdered metal is first compacted under high pressure into a shape that approximates the final product. The pressure causes the powdered particles to lock together mechanically, but the compacted product ("compact") retains significant porosity. To reduce porosity, the compact is further treated by heating in a process known as sintering, with or without the application of pressure. During sintering, a compact is subjected to sufficient heat that at least some part of the components of the powder enter into a liquid phase which fills the pores within the compact.
It has been recognized that when the powder was made by water atomization of a melt, it may have an undesirably high oxygen content. The presence of oxygen in combined form in the compact, or in the final product, has a deleterious effect resulting, for example, in inferior strength. U.S. Pat. No. 4,063,940 addresses the sintering of powdered metal compacts in a deoxygenated atmosphere to cause densification of the compact to at least 98% relative density. By deoxidizing the powder, higher sintering temperatures can be used, preferably in the range 1,050.degree.-1,200.degree. C., but below the solidus temperature of the alloy.
While the '940 patent addresses the issues of increased densification of powdered metal compacts, and more efficient deoxidization at higher temperatures, the patent does not address critical issues of ductility, strength and machinability of powdered metal products. Thus, for example, powdered metal products intended for precision applications must subsequently be machined to close tolerances for intended use. Because of the hardness of the sintered powdered metal products, machinability in terms of wear of the machine tool, becomes a significant factor. Moreover, for certain applications the product should retain a level of ductility that allows ordinary manipulation with minimal risk of breakage. Thus, for example, valve seat inserts for automotive applications that are fabricated through powdered metallurgical techniques must be precision machined and thereafter manipulated into position in an engine. From a production stand point, a reduction in machine tool wear would reduce the downtime associated with changing the cutting tool and allows better utilization of expensive capital equipment. Moreover, improved ductility would reduce the number of valve seats that must be rejected due to failures that arise from a lack of ductility.